One technique for providing high bit rates on a communication link employs two or more antennas at both sides of the link to transmit two or more parallel data streams. This technique is referred to as “multiple input multiple output” (MIMO). Using MIMO, the spectrum efficiency of a radio-link can be N-folded, where N is the number of Tx/Rx antennas.
A drawback for some MIMO systems is that channel quality information must be fed back to the network in order to provide information for a link adaptation algorithm. Traditionally, a channel quality indicator (CQI) is used to inform the network about the current channel quality. When MIMO is used. the quality of all the parallel streams is needed by the link adaptation algorithm. This will increase the feedback rate of the CQI and hence limit the performance, especially for power limited handheld terminals.
For some protocols (e.g., High-Speed Downlink Packet Access (HSDPA)), the CQI consists of a 5-bit word which represents the number of bits that can be received during a transmission time interval (TTI) (with a certain block error rate (BLER)). Hence, the mobile terminal (a.k.a., user equipment (UE)) measures a channel quality (e.g., signal to interference plus noise ratio (SINR), signal to noise ratio (SNR), or other measure of channel quality) and maps this to a table containing transport formats (TF). The index to a certain TF is then transmitted back to the network node as the CQI. The network node uses this CQI when deciding on a suitable transmission format. When MIMO is in use, the CQI per data stream is a 4-bit word corresponding to the quality of that data stream. This means that the CQI feedback is increased from 5 to 8-bits when two streams are in use.
For long term evolution (LTE) a similar scheme is used. But, in contrast to HSDPA, the CQI for the second stream is differentially encoded versus the first stream. This means that the quality reported for the second stream is the difference towards the first stream. As in HSDPA, the CQI for the first stream is represented by a 4-bit word, while the difference between the two streams is reported as a second CQI. Since the difference, in general, is smaller than the total dynamic range for one CQI, this can be represented by a 3-bit word. This means that we can save at least 1 bit for each CQI transmission.
In HSPA UL it is reasonable to assume that interference cancelling receivers will be in use in a high performance network because UL interference is the quantity that will limit the performance. If the impact from other users is removed, a much higher interference contribution can be tolerated, hence the capacity can be increased. It is therefore likely that interference cancellation techniques also will be applied to MIMO in UL, when introduced. However, designing a system towards one type of algorithm is not preferred because the technical evolution within receiver algorithms is fast, and better algorithms may be available in a not too distance future.
The differential CQI reporting approach described above assumes that the two CQIs are reasonably similar. For a linear receiver (e.g. an MMSE based receiver) the two streams will, on average, have the same SINR. The difference, in rate, between the two streams will then be rather small. Hence, the difference can be coded with a smaller number of bits than if the two CQIs were reported separately.
On the other hand, if a receiver structure based on inter-stream interference cancellation is used, then the two streams may experience very different SINR. For example, when using a successive interference canceller (SIC), where the influence of stream one is removed before detecting stream two, the SINR for stream two will be higher than if the interference was not removed. Accordingly, the mean rate of the two streams may become very different, hence the difference in reported CQI can be substantial. In this case, it is not possible to represent the difference with fewer bits than the individual CQIs. Thus, the differential CQI reporting approach will not be advantageous anymore.
Thus, there exists a need to overcome this above described problem.